How does humidity work? How can there be more moisture in the air in summer
than in winter when it is wetter weather?

Blair Blakie, a physicist researching ultra-cold gases at the
University of Otago, responded.

Humidity is a measure of the amount of water in air. Humidity
increases when water is added to the air through evaporation
and is reduced by cooling the air and causing the moisture
to condense (this is how a dehumidifier works). The most
commonly used measure of humidity is relative humidity; this
is the amount of water in the air as compared to a sample of air
(at the same temperature) that is saturated with water vapour.

Relative humidity is not so useful for comparing the water content of air
samples at different temperatures, because the saturation level changes
significantly with temperature. It takes much more water to saturate hot
air than cold air.

As an example of the seasonal conditions in New Zealand, the following table
contains NIWA data on the average temperature, humidity and rainfall in Auckland.

The higher rainfall during winter is enhanced by the high relative humidity, since
as the relative humidity reaches 100% vapour will tend to condense into liquid
water. However, despite having lower relative humidity, the warmer summer air has
more water vapour. This is revealed by comparing the absolute humidity: the mass of
water within a cubic metre of air. The absolute humidity is estimated in the last
row of the table, showing that summer air has roughly 40% more water vapour than winter.